Oximeter probe with light wavelengths to avoid surgical dyes

ABSTRACT

A tissue oximetry device utilizes at least three or at least four different wavelengths of light for collection of reflectance data where the different wavelengths are longer than 730 nanometers. The three or four wavelengths are utilized to generate a range of reflectance data suited for accurate determination of oxygenated hemoglobin and deoxygenated hemoglobin concentrations. The relatively long wavelengths decrease optical interference from certain dyes, particularly methylene blue and PVPI, which may be present on tissue being analyzed for viability and further enhance the generation of accurate reflectance data. The wavelengths are 760 nanometers, 810 nanometers, and 850 nanometers, or 760 nanometers, 810 nanometers, 850 nanometers, and 900 nanometers.

CROSS-REFERENCE TO RELATED APPLICATIONS

This patent application is a divisional of U.S. patent application Ser.No. 14/977,578, filed Dec. 21, 2015, issued as U.S. Pat. No. 10,335,069on Jul. 2, 2019, which is a continuation of U.S. patent application Ser.No. 13/887,178, filed May 3, 2013, issued as U.S. Pat. No. 9,216,000 onDec. 22, 2015, which claims the benefit of U.S. patent applications61/642,389, 61/642,393, 61/642,395, and 61/642,399, filed May 3, 2012,and 61/682,146, filed Aug. 10, 2012. These applications are incorporatedby reference along with all other references cited in this application.

BACKGROUND OF THE INVENTION

The present invention relates generally to optical systems that monitoroxygen levels in tissue. More specifically, the present inventionrelates to optical probes that include sources and detectors on sensorheads of the optical probes for emitting and detecting light.

Oximeters are medical devices used to measure oxygen saturation oftissue in humans and living things for various purposes. For example,oximeters are used for medical and diagnostic purposes in hospitals andother medical facilities (e.g., surgery, patient monitoring, orambulance or other mobile monitoring for, e.g., hypoxia); sports andathletics purposes at a sports arena (e.g., professional athletemonitoring); personal or at-home monitoring of individuals (e.g.,general health monitoring, or personal training for a marathon); andveterinary purposes (e.g., animal monitoring).

Pulse oximeters and tissue oximeters are two types of oximeters thatoperate on different principles. A pulse oximeter requires a pulse inorder to function. A pulse oximeter typically measures the absorbance oflight due to the pulsing arterial blood. In contrast, a tissue oximeterdoes not require a pulse in order to function, and can be used to makeoxygen saturation measurements of a tissue flap that has beendisconnected from a blood supply.

Human tissue, as an example, includes a variety of molecules that caninteract with light via scattering or absorption (e.g., vialight-absorbing chromophores). Such chromophores include oxygenated anddeoxygenated hemoglobins, melanin, water, lipid, and cytochrome.Oxygenated and deoxygenated hemoglobins are the most dominantchromophores in the spectrum range of 600 nanometers to 900 nanometers.Light absorption differs significantly for oxygenated and deoxygenatedhemoglobins at certain wavelengths of light. Tissue oximeters canmeasure oxygen levels in human tissue by exploiting theselight-absorption differences.

Despite the success of existing oximeters, there is a continuing desireto improve oximeters by, for example, improving measurement accuracy;reducing measurement time; lowering cost; reducing size, weight, or formfactor; reducing power consumption; and for other reasons, and anycombination of these.

In particular, assessing a patient's oxygenation state is important asit is an indicator of the state of the patient's health. Thus, oximetersare often used in clinical settings, such as during surgery andrecovery, where it may be suspected that the patient's tissueoxygenation state is unstable. For example, during surgery, oximetersshould be able to quickly deliver accurate oxygen saturationmeasurements under a variety of nonideal conditions. While existingoximeters have been sufficient for post-operative tissue monitoringwhere speed of measurement is less critical, existing oximetersfluctuate substantially and give inaccurate saturation measurements whenused during surgery where various elements can interfere with accuratereading, such as if the oximeter comes in contact with blood.

Therefore, there is a need for improved tissue oximetry probes andmethods of making measurements using these probes.

BRIEF SUMMARY OF THE INVENTION

A tissue oximetry device utilizes at least two different wavelengths oflight for collection of reflectance data where the wavelengths are above700 nanometers. Utilizing two, three, or four wavelengths generates arange of data that is suited for accurate determination of oxygenatedhemoglobin and deoxygenated hemoglobin concentrations.

According to one embodiment, a tissue oximetry device includes aprocessor; a memory coupled to the processor; and a plurality of lightsources. The light sources are controlled by the processor, and generateand emit at least two wavelengths of light longer than 700 nanometers.The tissue oximetry device further includes a plurality of detectorsconfigured to be controlled by the processor. The processor isconfigured to: control the plurality of light sources to generate andemit the light into tissue, control the plurality of detectors to detectthe light subsequent to reflection of the light from the tissue, controlthe plurality of detectors to generate reflectance data for the tissuebased on detection of the light by the plurality of detectors, anddetermine the oxygen saturation for the tissue based on the reflectancedata.

According to one specific embodiment, the at least two wavelengths areapproximately 760 nanometers and 850 nanometers. According to analternative specific embodiment, the plurality of light sources isconfigured to generate and emit at least three wavelengths of lighthaving wavelengths of 760 nanometers, 810 nanometers, and 850nanometers. According to another alternative specific embodiment, theplurality of light sources is configured to generate and emit at leastfour wavelengths of light having wavelengths of approximately 760nanometers, 810 nanometers, 850 nanometers, and 900 nanometers.

According to another embodiment, a tissue oximetry device includes aprocessor; a memory coupled to the processor; and a plurality of lightsources that are controlled by the processor. The light sources areconfigured to generate and emit at least two wavelengths of light thatare longer than wavelengths of primary absorption peaks of methyleneblue. The tissue oximetry device further includes a plurality ofdetectors configured to be controlled by the processor. The processor isconfigured to: control the plurality of light sources to generate andemit the light into tissue; control the plurality of detectors to detectthe light subsequent to reflection of the light from the tissue; controlthe plurality of detectors to generate reflectance data for the tissuebased on detection of the light by the plurality of detectors; anddetermine the oxygen saturation for the tissue based on the reflectancedata.

These relatively long wavelengths tend to decrease optical interferencewith certain dyes, particularly methylene blue and povidone-iodine(PVPI, e.g., Betadine® of Purdue Products L.P. of Stamford, Conn.),which may be present in tissue being analyzed for viability, and furtherenhances the generation of accurate reflectance data. The wavelengthsalso avoid gentian violet, which is often used in tissue marking pens.The wavelengths utilized by the tissue oximetry device are outside ofthe peak absorptive ranges of methylene blue, gentian violet, and PVPI.Therefore, relatively accurate reflectance data may be acquired in anincreased number of surgical situations than was acquired by tissueoximetry device utilizing other wavelengths. Further, the use of theseparticular two, three, or four different wavelengths provides sufficientreflectance data to solve the two-variable, three-variable, orfour-variable relations from which oxygenated hemoglobin anddeoxygenated hemoglobin concentrations can be determined, depending onhow many additional tissue chromophores are included (e.g., melanin, orothers). The utilization of optimal probing wavelengths improves tissueoximetry device performance in intraoperative situations involving dyesas compared to the tissue oximetry devices considered to be prior art.

Other objects, features, and advantages of the present invention willbecome apparent upon consideration of the following detailed descriptionand the accompanying drawings, in which like reference designationsrepresent like features throughout the figures.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an absorption graph that shows the absorption coefficient ofmethylene blue and PVPI at wavelengths ranging from 500 nanometers tojust below 750 nanometers and shows the predominant absorption ofwavelengths below 700 nanometers for methylene blue.

FIG. 2 is an absorption graph that shows the absorption coefficient ofmethylene blue at wavelengths ranging from 700 nanometers to 900nanometers.

FIG. 3 is a simplified image of a tissue oximetry device according toone embodiment.

FIG. 4A is a simplified end view of the tissue oximetry probe accordingto one embodiment.

FIG. 4B is a simplified end view of the tissue oximetry probe accordingto an alternative embodiment.

FIG. 5 is a block diagram of the tissue oximetry device according to oneembodiment.

DETAILED DESCRIPTION OF THE INVENTION

Colored dyes often are present on or have been absorbed by the tissueregions that clinicians wish to check for viability. Methylene blue isone dye that is often used for sentinel lymph node biopsies, which areoften performed during the same surgical session as a mastectomy inorder to determine the degree to which cancerous tissue may have spread.Therefore, methylene blue can be present in the tissue being analyzedfor viability for reconstruction or the like. Methylene blue absorbslight readily in the 500 nanometer to 700 nanometer range with theabsorption tailing off at about 730 nm.

Povidone-iodine (PVPI, e.g., Betadine® of Purdue Products L.P. ofStamford, Conn.) is an orange dye that is often used as an antisepticprior to making surgical incisions and may therefore also be present ontissue of interest. Similar to methylene blue, PVPI absorbs lightreadily in the 500 nanometer to 700 nanometer range, however to a lesserdegree than methylene blue. Further, gentian violet is a dye that isoften used in tissue marking pens, such as the pens used by plasticsurgeons and may therefore be present on tissue of interest.

FIGS. 1 and 2 are absorption graphs that show the absorption coefficientμa of methylene blue at wavelengths ranging from 500 nanometers to 900nanometers. FIG. 1 also shows the predominant absorption by methyleneblue of wavelengths below 700 nanometers and shows the primaryabsorption peaks of methylene blue centered at about 600 nanometers and660 nanometers. FIG. 1 also shows the absorption coefficient μa of PVPIat wavelengths ranging from 500 nanometers to just below 750 nanometers.FIG. 1 also shows the predominant absorption by PVPI of wavelengthsbelow 650 nanometers and shows the primary absorption peak of PVPIcentered at about 510 nanometers.

The presence of methylene blue, PVPI, or other dyes can interfere withthe determinations of tissue viability. For example, surgeons may usetissue oximetry devices for determining the viability of tissue, anddyes present on the tissue can absorb the wavelengths used by the tissueoximetry devices for providing tissue viability information.

FIG. 3 is a simplified image of a tissue oximetry device 100 accordingto one embodiment. Tissue oximetry device 100 is configured to maketissue oximetry measurements, such as intraoperatively andpostoperatively. In an implementation, the tissue oximetry device ishandheld and can make tissue oximetry measurements and display thesemeasurements, without needing to connect to another external componenteither via a cable or wirelessly. The electronics to make measurementsand calculations is contained entirely within the housing of the tissueoximetry device. The tissue oximetry device is a standalone handheldtissue oximeter device, without a cable or wireless connection.

Tissue oximetry device 100 may be a handheld device that includes atissue oximetry probe 115 (also sometimes referred to as a sensor head),which may be positioned at an end of a sensing arm 114. Tissue oximetrydevice 100 is configured to measure the oxygen saturation of tissue byemitting light, such as red and near-infrared light, from tissueoximetry probe 115 into tissue, and collecting light reflected from thetissue at the tissue oximetry probe.

Tissue oximetry device 100 may include a display 112 or othernotification device that notifies a user of oxygen saturationmeasurements made by the tissue oximetry device. While tissue oximetryprobe 115 is described as being configured for use with tissue oximetrydevice 100, which is a handheld device, tissue oximetry probe 115 may beused with other tissue oximetry devices, such as a modular tissueoximetry device where the tissue oximetry probe is at the end of a cabledevice that connects to a base unit. The cable device might be adisposable device that is configured for use with a single patient andthe base unit might be a device that is configured for repeated use.Such modular tissue oximetry devices are well understood by those ofskill in the art and are not described further.

FIG. 4A is a simplified end view of tissue oximetry probe 115 accordingto one embodiment. Tissue oximetry probe 115 is configured to contacttissue (e.g., a patient's skin) for which a tissue oximetry measurementis to be made. Tissue oximetry probe 115 includes a set of light sources120 (generally light sources 120) and includes a set of detectors 125(generally detectors 125). The set of light sources 120 may include twoor more light sources. According to the embodiment shown in FIG. 4A,tissue oximetry probe 115 includes three light sources 120 a, 120 b, and120 c, but may alternatively include two light sources, such as lightsources 120 a and 120 c where light source 120 b is omitted. Additionallight sources (not shown) can be added. FIG. 4B is a simplified end viewof a tissue oximetry probe 115′ according to an embodiment where thetissue oximetry probe includes the two light sources 120 a and 120 c,but does not include light source 120 b. Aside from the different numberof light sources, tissue oximetry probes 115 and 115′ are substantiallysimilar.

The set of detectors 125 may include eight detectors 125 a, 125 b, 125c, 125 d, 125 e, 125 f, 125 g, and 125 h as shown, but may include moreor fewer detectors. Detectors 125 are positioned with respect to outerlight sources 120 a and 120 c such that eight or more (e.g., fourteen)unique source-to-detector distances are created. The shortestsource-to-detector distances may be the same. For example, the shortestsource-to-detector distance D1 between light source 120 a and detector125 e, and the shortest source-to-detector distance D2 between lightsource 120 c and detector 125 a may be the same. It follows that thesource-to-detector distance D3 between light source 120 a and detector125 a, and the source-to-detector distance D4 between light source 120 cand detector 125 e may also be the same. The source-to-detectordistances D3 and D4 are the longest source-to-detector distance forlight sources 120 a and 120 c. With the exception of the shortestsource-to-detector distance and the longest source-to-detector distancefor light sources 120 a and 120 c, the source-to-detector distances forlight sources 120 a and 120 c may be unique. As described above, tissueoximetry probe 115 may have fourteen unique source-to-detector distancesthat allow for fourteen reflectance data points to be collected bydetectors 125 from each wavelength of light emitted from light sources120. As described in further detail below, each light source 120 isconfigured to generate and emit a number of wavelengths.

Detectors 125 are solid state detectors and may be mounted on a printedcircuit board (PCB, not shown), which routes various signal to and fromthe detectors. Further, detectors 125 may be combined devices ordiscrete devices.

While the tissue oximetry probes 115 and 115′ are described above ashaving circularly arranged detectors, the detectors may be positioned inother arrangements, such as linear, triangular, rectangular, square, andothers. In some embodiments, the light sources may also be alternativelyarranged, such as in a triangular arrangement, a rectangulararrangement, and others.

In a specific implementation, detectors 125 are positioned with respectto outer light sources 120 a and 120 c such that four or more (e.g.,fourteen) unique source-to-detector distances are created. With greaternumbers of source-to-detector distances, this can be used to obtaingreater accuracy, faster calibration, and redundancy (when duplicatesource-to-detector distances are provided). At least twosource-to-detectors distances are about 1.5 millimeters or less (e.g.,0.5 millimeters up to about 1.7 millimeters), and at least two more twosource-to-detectors distances are about 2.5 millimeters or greater(e.g., 1.5 millimeters up to about 3.2 millimeters).

In other words, a first source-to-detector distance is about 1.5millimeters or less. A second source-to-detector distance is about 1.5millimeters or less. A third source-to-detector distance is about 2.5millimeters or greater. A fourth source-to-detector distance is about2.5 millimeters or greater. There can be various numbers of sources anddetector arrangements to obtain these four source-to-detector distances,such as one source and four detectors, two sources and two detectors,one detector and four sources, or other arrangements and combinations.

For example, an implementation includes at least two sources and atleast two detectors, where a maximum distance between a source and adetector is about 4 millimeters (or about 5 millimeters). At least twosource-to-detector are about 2.5 millimeters or greater. At least twosource-to-detector distances are about 1.5 millimeters or less.

When a greater number of sources and detectors are used, greater numbersof source-to-detector distances are available. As discussed, these canbe used to provide greater accuracy, faster calibration, or redundancy,or a combination. The arrangement of the sources and detectors can be incircular pattern, such as at points along the arc of a circle withradius (e.g., 4 millimeters, or 5 millimeters). In an implementation, atolerance of the detector or source positions on the arc is within 10microns of the arc curve. In other implementations, the tolerance iswithin about 0.5 millimeters.

Wavelengths Generated and Emitted From the Light Sources

Each light source 120 may include a fiber optic cable and one or morelight emitting diodes (LEDs) or laser diodes (generally wavelengthsources) that transmit generated light into the fiber optic cable. Forexample, each light source 120 may include two or more wavelengthsources that generate two or more substantially unique wavelengths. Thewavelengths may all be longer than 730 nanometers, e.g., in the red andnear infrared.

According to an embodiment where each light source 120 includes twowavelength sources, the wavelength sources may be configured to generateand emit wavelengths of approximately 760 nanometers (e.g., +/−10nanometers), and 850 nanometers (e.g., +/−20 nanometers). According toan embodiment where each light source 120 includes three wavelengthsources, the wavelength sources may be configured to generate and emitwavelengths of approximately 760 nanometers (e.g., +/−10 nanometers),810 nanometers (e.g., +/−10 nanometers), and 850 nanometers (e.g., +/−20nanometers). According to another embodiment, where each light source120 includes four wavelength sources, the wavelength sources may beconfigured to emit wavelengths of approximately 760 nanometers (e.g.,+/−10 nanometers), 810 nanometers (e.g., +/−10 nanometers), 850nanometers (e.g., +/−20 nanometers), and 900 nanometers (e.g., +/−20nanometers). Additional and/or alternative wavelengths may be utilizedby tissue oximetry device 100.

Use of the described wavelengths by tissue oximetry device 100 tends todecrease the fraction of emitted light that can be absorbed by methyleneblue, gentian violet, and PVPI, and thereby increases the fraction oflight that can be scattered or absorbed by intrinsic tissue elements andgenerates accurate reflectance data. Accurate reflectance data isnecessary in order to extract the optical properties of tissue fromwhich the concentrations of oxygenated and deoxygenated hemoglobin canbe derived.

For the foregoing described wavelengths, tissue scattering is relativelylow and light penetrates farther into tissue than shorter wavelengths.Further, the foregoing described wavelengths are on both sides of anoxygenated-deoxygenated hemoglobin spectral crossing point called anisosbestic point, which is 810 nanometers for hemoglobin. As such, whenone chromophore (e.g., oxygenated hemoglobin) has high absorption, theother chromophore (e.g., deoxygenated hemoglobin) then has lowabsorption and vice versa. The tissue oximetry device's utilization ofwavelengths surrounding the isosbestic point provides for relativelyimproved statistics for oxygen saturation determinations.

In at least one of the foregoing described embodiments, tissue oximetrydevice 100 utilizes a wavelength at approximately the isosbestic point,at 810 nanometers. At the isosbestic point the absorption of the 810nanometer wavelength for oxygenated hemoglobin and deoxygenatedhemoglobin are equivalent and therefore provides a stable referencepoint in the reflectance data generated by detectors 125. Relativelylonger wavelengths, such as the 900 nanometer wavelength of at least oneembodiment allows for distinguishing between the absorption curve fordeoxygenated hemoglobin from the absorption curve for melanin.

Tissue Oximetry Device Circuit

FIG. 5 is a block diagram of tissue oximetry device 100 according to oneembodiment. Tissue oximetry device 100 according to the embodiment shownin FIG. 5 includes display 112, a processor 505, a memory 510, a speaker518, one or more user-selection devices 519 (e.g., one or more switchesfor initiating oxygen saturation measurements), the set of light sources120, the set of detectors 125, and a power source (e.g., a battery) 527.The foregoing listed components may be linked together via a bus 528,which may be the system bus architecture of tissue oximetry device 100.Although this figure shows one bus that connects to each component, thebusing is illustrative of any interconnection scheme serving to linkthese components or other components included in tissue oximetry device100 subsystems. For example, speaker 518 (e.g., an alternative devicefor notifying a user of oxygen saturation measurements) could beconnected to a subsystem through a port or have an internal directconnection to processor 516. Further, the components described arehoused in a mobile housing (see FIG. 1) of tissue oximetry device 100according to at least one embodiment.

Processor 505 may include a microprocessor, a microcontroller, controllogic, a multi-core processor, or the like. Further, processor 505 maycontrol turning on and turning off the wavelength sources as describedbelow. Memory 510 may include a variety of memories, such as a volatilememory 510 a (e.g., a RAM), a nonvolatile memory 519 b (e.g., a disk,Flash memory, electrically erasable memory, PROM, and others). Memory510 may collect and store reflectance data generated by detectors 125.Different implementations of tissue oximetry device 100 may include anynumber of the listed components, in any combination or configuration,and may also include other components not shown.

Power source 127 can be a battery, such as a disposable battery.Disposable batteries are discarded after their stored charge isexpended. Some disposable battery chemistry technologies includealkaline, zinc carbon, or silver oxide. The battery has sufficientstored charged to allow use of the handheld device for several hours.After use, the handheld unit is discarded.

In other implementations, the battery can also be rechargeable where thebattery can be recharged multiple times after the stored charge isexpended. Some rechargeable battery chemistry technologies includenickel cadmium (NiCd), nickel metal hydride (NiMH), lithium ion(Li-ion), and zinc air. The battery can be recharged, for example, viaan AC adapter with cord that connects to the handheld unit. Thecircuitry in the handheld unit can include a recharger circuit (notshown). Batteries with rechargeable battery chemistry may be sometimesused as disposable batteries, where the batteries are not recharged butdisposed of after use.

Use of Wavelengths for Optical Probing

Oxygenated and deoxygenated hemoglobin concentrations, from which oxygensaturation can be calculated, can be related to the absorptioncoefficient μa of a region of tissue for a given wavelength of light. Insome cases, a simple relationship is used for calculation where theabsorption coefficient is assumed to depend only on the concentrationsof oxygenated and deoxygenated hemoglobin. However, melanin and waterpresent in tissue can also absorb incident light so this simplerelationship may be insufficient for highly accurate concentrationcalculations, as absorption from water and melanin may be incorrectlyattributed to oxygenated or deoxygenated hemoglobin. A relationshipbetween the absorption coefficient and the concentrations of oxygenatedhemoglobin (HbO2), deoxygenated hemoglobin (Hb), water (H2O), andmelanin (mel) may be:μ_(a)=2.303(ε_(HbO2)[HbO2]+ε_(Hb)[Hb]+ε_(H2O)[H2O]ε_(mel)[mel])

where ε_(species) denotes the molar absorptivity of a given species andbracketed quantities indicate concentration values.

The shape of a reflectance curve (generated by plotting the intensity ofdiffusely reflected or re-emitted light) can be analyzed to obtain theabsorption and scattering coefficients for a given region of tissue.There are four unknown concentrations (i.e., [HbO2], [Hb], [H2O], and[mel]) in the above relationship that correspond to the absorptioncoefficient. Once the absorption coefficient is determined for a givenwavelength, the relationship becomes an equation of four unknownvariables. However, since the concentrations of oxygenated anddeoxygenated hemoglobin, water, and melanin should not vary considerablyover the course of a probe measurement, probing the tissue with fourdifferent wavelength emitted by the wavelength sources can provide fourvalues for μa, which can be used to determine the four relevantconcentrations in the expression for μa. That is, a system of fourequations with four unknown variables can be solved, as is wellunderstood. From the determined concentrations of oxygenated hemoglobins[HbO2] and deoxygenated hemoglobins [Hb], the oxygen saturation oftissue can be determined.

According to the embodiment where three wavelengths are emitted by thewavelength sources, the contributions from water, melanin, and otherlight absorbers can be combined into a single term and expressed as:μ_(a)=2.303(ε_(HbO2)[HbO2]+ε_(Hb)[Hb]+ε_(H2O,mel)[H2O,mel]).

If three absorption coefficients μ_(a) are determined for the threewavelengths, then the three relevant concentrations for [HbO2], [Hb],and [H2O,mel]) can be determined, and the oxygen saturation can again bedetermined from the determined concentrations of oxygenated anddeoxygenated hemoglobins. The absorption coefficients may be determinedfrom the reflectance data by a variety of methods, such as fitting thereflectance data to one or more predetermined reflectance curves, whereeach predetermined reflectance curve represents a unique absorptioncoefficient. The absorption coefficients may alternatively be determinedby vector multiplication with the net analyte signal, which is describedin U.S. Pat. No. 6,597,931, titled “System and Method for AbsoluteOxygen Saturation,” and is incorporated by reference.

Wavelength Source Control

The wavelength sources may be cycled on and off at a variety offrequencies. For example, the wavelength sources may be turned on insequence with one wavelength source on at any one time. The wavelengthsources may be cycled at 30 hertz. Additionally each wavelength sourcemay be modulated at a variety of frequencies in order to reject ambientlight. For example, each wavelength source may be individually modulatedat 2.5 kilohertz. Further, the wavelength sources may be individuallycycled in a specific order. Detectors 125 may be substantiallycontinuously monitored as the wavelength sources are cycled. Processor505 may control the cycle order of the wavelength sources. Based on thecycle order, the reflectance data collected by detectors 125 may beappropriately categorized according to wavelength based on the knowncycling of the wavelength sources on and off. The reflectance data maybe stored in memory 510 for use by processor 505 in determining theoxygenated and deoxygenated hemoglobin concentrations and to furtherdetermine the oxygen saturation of tissue being probed.

This description of the invention has been presented for the purposes ofillustration and description. It is not intended to be exhaustive or tolimit the invention to the precise form described, and manymodifications and variations are possible in light of the teachingabove. The embodiments were chosen and described in order to bestexplain the principles of the invention and its practical applications.This description will enable others skilled in the art to best utilizeand practice the invention in various embodiments and with variousmodifications as are suited to a particular use. The scope of theinvention is defined by the following claims.

The invention claimed is:
 1. A method comprising: providing a sensorhead for a tissue oximetry device comprising: a plurality of detectorstructures arranged symmetrically in a symmetric arrangement, symmetricabout a point on a line passing through a closed shape of the symmetricarrangement; a first source structure at a first point of the closedshape of the symmetric arrangement, wherein the first source structureis configured to emit at least a first wavelength of light longer thanabout 730 nanometers; a second source structure positioned at a secondpoint of the closed shape of the symmetric arrangement, wherein thefirst and second source structure are positioned on a line on the sensorhead, and the second source structure is configured to emit at least asecond wavelength of light longer than about 730 nanometers; a firstdetector structure on the closed shape of the detector structures,wherein a first distance is from the first detector structure to thefirst source structure, a second distance is from the first detectorstructure to the second source structure, and the first distance isgreater than the second distance; a second detector structure on theclosed shape of the detector structures, wherein a third distance isfrom the second detector structure to the first source structure, afourth distance is from the second detector structure to the secondsource structure, and the fourth distance is greater than the thirddistance; a third detector structure on the closed shape of the detectorstructures, wherein a fifth distance is from the third detectorstructure to the first source structure, a sixth distance is from thethird detector structure to the second source structure, the fifthdistance is different from the first distance and the second distance,and the sixth distance is different from the first distance and thesecond distance; and a fourth detector structure on the closed shape ofthe detector structures, wherein the first, second, third, and fourthdetectors structures are not on the line, a seventh distance is from thefourth detector structure to the first source structure, an eighthdistance is from the fourth detector structure to the second sourcestructure, the seventh distance is different from the first, second, andfifth distances, and the eighth distance is different from the first,second, and sixth distances; and determining an oxygen saturation valuefor a tissue to be measured using the first wavelength of light from thefirst source structure and the second wavelength of light from thesecond source structure emitted into the tissue and correspondingreflected light received by at least two of the detector structures. 2.The method of claim 1 wherein the determining an oxygen saturation valuefor a tissue comprises: receiving digital reflectance data for thereflected light received by the at least two of the detector structures;calculating absorption coefficients using the digital reflectance data;solving a set of reflection coefficient equations for the tissue to bemeasured using the absorption coefficients to determining concentrationvalues of at least oxygenated hemoglobin and deoxygenated hemoglobin;and determining an oxygen saturation value for the tissue using theconcentration values of oxygenated hemoglobin and deoxygenatedhemoglobin.
 3. The method of claim 1 wherein the first wavelength oflight is at least one of 760 nanometers, 810 nanometers, 850 nanometers,or 900 nanometers.
 4. The method of claim 1 wherein the first wavelengthof light is at least one of 760 nanometers, 810 nanometers, or 850nanometers, and the second wavelength of light is at least one of 760nanometers, 810 nanometers, 850 nanometers, or 900 nanometers.
 5. Themethod of claim 1 wherein the tissue oximetry device is a handheldtissue oximetry device comprising the housing comprising a processor,memory, display, and battery, and the sensor head is coupled to thehousing.
 6. The method of claim 1 wherein an enclosure of the tissueoximetry device comprises the sensor head, the sensor head is at an endof the tissue oximetry device, and when the sensor head is placedagainst the tissue to be measured, a display of the tissue oximetrydevice faces a user, and the display is coupled to the enclosure.
 7. Themethod of claim 1 wherein the first and second source structures arecoupled via waveguides to light emitting diodes.
 8. The method of claim1 wherein the first and second source structures are coupled via opticalfibers to light emitting diodes.
 9. The method of claim 1 wherein thefirst and second source structures comprise light emitting diodes. 10.The method of claim 1 wherein the first, second, third, and fourthdetector structures are coupled via waveguides to photodetectors. 11.The method of claim 1 wherein the first, second, third, and fourthdetector structures are coupled via optical fibers to photodetectors.12. The method of claim 1 wherein the first, second, third, and fourthdetector structures comprise photodetectors.
 13. The method of claim 1wherein the first and second source structures are coupled via opticalfibers to light emitting diodes, and the first, second, third, andfourth detector structures comprise photodetectors.
 14. The method ofclaim 1 wherein the first and second source structures are coupled viawaveguides to light emitting diodes, and the first, second, third, andfourth detector structures comprise photodetectors.
 15. The method ofclaim 1 wherein the closed shape is a circle.
 16. The method of claim 1wherein the first and second source structures are not between the firstand second detector structures.
 17. A method comprising: providing asensor head for a tissue oximetry device comprising: a plurality ofdetector structures arranged symmetrically in a symmetric arrangement,symmetric about a point on a line passing through the symmetricarrangement; a first source structure at a first point of the symmetricarrangement, wherein the first source structure is configured to emit atleast a first wavelength of light longer than about 730 nanometers; asecond source structure positioned at a second point of the symmetricasymmetric arrangement, wherein the first and second source structureare positioned on a line on the sensor head, and the second sourcestructure is configured to emit at least a second wavelength of lightlonger than about 730 nanometers; a first detector structure of thedetector structures, wherein a first distance is from the first detectorstructure to the first source structure, a second distance is from thefirst detector structure to the second source structure, and the firstdistance is greater than the second distance; a second detectorstructure of the detector structures, wherein the first and seconddetector structures are not on the line, the first and second sourcestructures are not between the first and second detector structures, athird distance is from the second detector structure to the first sourcestructure, a fourth distance is from the second detector structure tothe second source structure, and the fourth distance is greater than thethird distance; and determining an oxygen saturation value for a tissueto be measured using the first wavelength of light from the first sourcestructure and the second wavelength of light from the second sourcestructure emitted into the tissue and corresponding reflected lightreceived by at least two of the detector structures.
 18. The method ofclaim 17 comprising: a third detector structure of the detectorstructures, wherein a fifth distance is from the third detectorstructure to the first source structure, a sixth distance is from thethird detector structure to the second source structure, the fifthdistance is different from the first distance and the second distance,and the sixth distance is different from the first distance and thesecond distance.
 19. The method of claim 18 comprising: a fourthdetector structure of the detector structures, wherein a seventhdistance is from the fourth detector structure to the first sourcestructure, an eighth distance is from the fourth detector structure tothe second source structure, the seventh distance is different from thefirst, second, and fifth distances, and the eighth distance is differentfrom the first, second, and sixth distances.
 20. The method of claim 17wherein the determining an oxygen saturation value for a tissuecomprises: receiving digital reflectance data for the reflected lightreceived by the at least two of the detector structures; calculatingabsorption coefficients using the digital reflectance data; solving aset of reflection coefficient equations for the tissue to be measuredusing the absorption coefficients to determining concentration values ofat least oxygenated hemoglobin and deoxygenated hemoglobin; anddetermining an oxygen saturation value for the tissue using theconcentration values of oxygenated hemoglobin and deoxygenatedhemoglobin.
 21. The method of claim 17 wherein the symmetric arrangementis a circular arrangement.